Organic light emitting diode (OLED) display devices are often characterized as having good linearity between a device current and device luminance. As a result, it is generally easy to adjust the luminance by controlling the device current. In driving an OLED display device, two approaches are typically used: an active matrix approach and a passive matrix approach. In the active matrix approach, the luminance is adjusted by controlling a voltage or a current of the device. In a passive matrix approach, the luminance is adjusted by controlling a duty ratio of the driving signal. The image data is proportional to the light intensity, but the image display device typically has a non-linear relationship between the input image data (gray scale input) and the output luminance. In many displays, the relationship between the gray scale input and the luminance that is output is an exponential relationship where the exponent is called gamma. If the gamma value is not set correctly, then there may be distortion in the displayed image resulting in degradations in the image quality. Different display devices typically have different optimal gamma values such that each device adjusts the gamma to optimize the image quality on the display. In OLED displays, the driving circuit controls the driving current of the driving transistor.
FIG. 1 is a block diagram of a conventional data driver circuit 20 for a display that provides gamma correction. The conventional data driver circuit 20 includes a controller 21, a lookup table 22, and a plurality of digital-to-analog converters (DACs) 23. The lookup table includes corrected gamma values for the red (R), green (G), and blue (B) color intensities for each pixel. The controller 21 receives video or color data 27 and a look-up table setting command 29 from an external controller 25, and receives 8-bit corrected gray scale data 26 from the lookup table 22. The 6-bit color data 27 are assigned to the respective R, G, and B colors to display 260K colors. The controller 21 receives the 6-bit color data and the 8-bit lookup table setting command and provides the 6-bit color data as an input 24 to the lookup table 22. In response, the lookup table 22 provides gamma corrected 8-bit gray scale data 26 to the controller 21. The 8-bit gray scale data 26 are generated by gamma correcting the 6-bit color data 24 and adding two additional dummy bits. The 8-bit gray scale data 26 are provided to the DACs 23, which generate the data current IDATA in response thereto.
Unfortunately, the lookup table 22 and the DACs 23 take up a relatively large amount of chip area and are relative complex due, at least in part, to the two additional dummy bits used in the 8-bit gray scale data 26. Moreover, the mapping data contained in the lookup table 22 may need to be refreshed periodically to account for noise in the system.